The aim of the present study was to investigate the influence of classic and atypical neuroleptics on the activity of rat CYP2D measured as a rate of ethylmorphine O-deethylation. The reaction was studied in control liver microsomes in the presence of neuroleptics, as well as in microsomes of rats treated intraperitoneally (i.p.) for 1-day or 2-weeks (twice a day) with pharmacological doses of the drugs (promazine, levomepromazine, thioridazine, perazine 10 mg kg(-1); chlorpromazine 3 mg kg(-1); haloperidol 0.3 mg kg(-1); risperidone 0.1 mg kg(-1); sertindole 0.05 mg kg(-1)), in the absence of the neuroleptics in vitro. Neuroleptics added in vitro to control liver microsomes decreased the activity of the rat CYP2D by competitive or mixed inhibition of the enzyme. Thioridazine (Ki=15 microM) was the most potent inhibitor of the rat CYP2D among the drugs studied, whose effect was more pronounced than that of the other neuroleptics tested: phenothiazines (Ki=18-23 microM), haloperidol (Ki=32 microM), sertindole (Ki=51 microM) or risperidone (Ki=165 microM). The investigated neuroleptics-when given to rats in vivo-also seemed to exert an inhibitory effect on CYP2D via other mechanisms. One-day exposure of rats to the classic neuroleptics decreased the activity of CYP2D in rat liver microsomes. After chronic treatment with the investigated neuroleptics, the decreased CYP2D activity produced by the phenothiazines was still maintained, while that caused by haloperidol diminished. Moreover, risperidone decreased the activity of that enzyme. The obtained results indicate drug- and time-dependent interactions between the investigated neuroleptics and the CYP2D subfamily of rat cytochrome P-450, which may proceed via different mechanisms: (1) competitive or mixed inhibition of CYP2D shown in vitro, the inhibitory effects of phenothiazines being stronger than those of haloperidol or atypical neuroleptics, but weaker than the effects of the respective drugs on human CYP2D6; (2) in vivo inhibition of CYP2D, produced by both 1-day and chronic treatment with phenothiazines, which suggests inactivation of enzyme by intermediate metabolites; (3) in vivo inhibition of CYP2D by risperidone, produced only by chronic treatment with the drug, which suggests its influence on the enzyme regulation.